The New Zealand Meteorological Service is responsible for providing weather forecasts and climato-logical information throughout New Zealand and its island territories (including the Cook Islands) and, by arrangement with the other governments concerned, also in the area covered by the Gilbert and Ellice Islands, Fiji, and Tonga. The headquarters and chief forecasting and analysis centre is located at Kelburn, Wellington. Forecasts for aircraft on overseas trunk routes are supplied from the offices at Auckland and Nandi, Fiji, while the Nandi office also provides an extensive hurricane-warning scheme. Forecasting offices are also located at Ohakea, Wellington, and Christchurch aerodromes, mainly for aviation purposes.
For almost a century the principal tool of the weather forecaster has been the synoptic chart or, as it is usually called, the weather map. Drawn every six hours, these maps are based on reports received from about 200 stations over an area extending from the equator to Antarctica, and from western Australia to Pitcairn Island. By international agreement each country maintains a network of weather reporting stations which forward coded reports to a national collecting centre. The reports contain details of cloud, visibility, and weather conditions, wind speed and direction, and atmospheric pressure (at mean sea level) and its change in the previous three hours. Also reported are air temperature and dew-point, rainfall, and the state of the sea (from coastal stations). Ships at sea also forward six-hourly reports to the nearest collecting centre, while hourly reports are contributed by aircraft on trans-ocean flights. Every six hours there is a rapid international exchange of information between collection centres using radio-teletype and facsimile transmissions. The basic observations thus assembled are plotted on the weather map in a kind of meteorological shorthand, and then the forecaster draws the isobars (joining places with equal mean-sea-level pressure) and the fronts (the boundaries along which large air-masses of different temperature converge).
The importance of the weather map arose from an early recognition of the existence of large-scale weather systems closely related to the pattern of the isobars. It was found that over areas of relatively high pressure (anticyclones, ridges), the weather is mainly fine, while the low pressure systems (depressions, troughs, cyclones) are associated with strong winds and unsettled weather. The wind flow follows the direction of the isobars, with low pressure on the right (in the southern hemisphere), and is strongest where the isobars are closest together. Since these pressure systems tend to change rather slowly and to retain their identity for some days, their progress can be followed from map to map. This serves as a rough basis for weather prediction. Serious errors will arise, however, unless changes in movement and intensity of the weather systems can be anticipated.
The above weather maps show the meteorological situations associated with two characteristic New Zealand weather phenomena.
In New Zealand the first weather maps were drawn by Capt. R. A. Edwin who was appointed to the Marine Department in 1874 to provide weather forecasts and storm warnings for shipping. These maps were based on reports from about two dozen stations telegraphed daily (except Sunday) to Wellington. No reports came from outside New Zealand but, following an Inter-Colonial Meteorological Conference in 1879, code numbers were assigned to 24 typical isobaric patterns commonly occurring over New Zealand and 20 over south-eastern Australia. The appropriate code numbers were exchanged daily by cable between the two countries and were also supplied to the main daily newspapers as a guide to the selection of the correct block for printing a daily weather map. Because of such scanty and infrequent reports, and the absence of information from the surrounding oceans, no great accuracy in forecasting was achieved, nor could it be expected. Yet for the next 40 years little change took place, either in the day-to-day collection of reports, or in the technique of forecasting.
In 1909 Rev. D. C. Bates succeeded Edwin as director of the Meteorological Office. At that time radio was in its infancy, but arrangements were soon made to receive weather reports from ships at sea. The intervention of the First World War delayed this project and it was not until 1919 that the flow of ships' reports began in earnest. In the same year the Norwegians introduced the concept of the polar front and the wave theory of cyclonic formation. This was a most important advance, as it offered for the first time a physical explanation for many of the developments which take place in middle latitudes; in particular, the tendency for the cloud and rain accompanying a depression to be arranged in bands rather than uniformly around the low-pressure centre. The Norwegian methods were first applied in New Zealand by Dr E. Kidson, and later extended by C. E. Palmer whose paper “Synoptic Analysis over the Southern Ocean” (1942) became the standard guide for meteorologists in this region for many years. Kidson directed the Meteorological Service from 1927 until his death in 1939.
In recent years meteorological services everywhere have expanded considerably, and meteorology as a science may be said to have come of age. Following the inauguration of regular air services in New Zealand in 1935, the number of reporting stations and the frequency of reports were greatly increased, while measurements of upper winds were introduced at selected stations, using small hydrogen-filled balloons tracked by theodolite. This method was effective only in the absence of low cloud, but the more recent use of radar for tracking enables the winds to be measured up to 50,000 ft and higher in all weather conditions. It also became possible to measure the physical properties of the atmosphere through the development of reasonably priced, lightweight radiosondes which automatically transmit pressure, temperature, and humidity while carried aloft by balloon. Daily radiosonde ascents commenced at Auckland in 1942; the present network consists of seven radiosonde and nine radiowind stations. As large rain-drops are able to reflect ultra-short radio waves, radar equipment is also used for locating and tracking areas of heavy rain, such as fronts and thunderstorms.
Regular upper-air sounding have enabled meteorologists for the first time to study, in their true three-dimensional setting, the physical and dynamical processes of the atmosphere which produce what we recognise as weather. Although the surface weather map has lost none of its former importance, it is now supplemented with several upper-level charts showing the height contours of the 700, 500, and 300 millibar surfaces, similar to isobars at about 10,000, 18,000, and 30,000 feet, and bearing a similar relationship to the wind flow. A thickness chart is also drawn for the layer 1,000–500 millibars.
The air flow at high levels is usually quite different from that at sea level; at times, it may even be almost opposite in direction. The upper-level charts reveal that many of the pressure systems that appear at the earth's surface are relatively shallow. On the other hand, the circulation of air around a depression may sometimes be stronger at high levels than at low levels. Narrow belts of extremely strong winds are often found in the 30,000–40,000 ft layer. Within these “jet-streams”, as they are called, winds often exceed 100 knots and may reach 200 knots or more. These naturally create a special problem in forecasting winds for high-flying aircraft.
The differences between the flow at low and high levels in the atmosphere often help to explain features of the weather that are not adequately made clear by the sea-level analysis. Furthermore, a number of semi-empirical rules have been found, relating the contour patterns on the upper-level and thickness charts to future changes in the weather systems. For example, a shallow system usually moves in the general direction of the winds blowing over it at high levels. Such rules are applied by the forecaster in the preparation of a “prognostic chart” representing the weather map as he expects it to appear, usually 24 hours ahead. In parts of the Northern Hemisphere, where upper-air stations are much more numerous, prognostic charts are now prepared automatically with the aid of large, very fast, electronic computers. The computer takes the latest observations and rapidly performs many thousands of intricate calculations to produce the expected values for the required period ahead. Even a computer can, at present, solve only an approximate form of the dynamical and physical equations, and the resulting prognostic map cannot be perfectly accurate. Moreover, the human forecaster is still required to bridge a very large gap in translating the prognostic map into terms of expected weather. Numerical prediction methods have been tried experimentally in New Zealand with promising results, but lack of upper-air data from the surrounding oceans, and restricted computer facilities, at present limit application on a routine basis.
Further developments in numerical prediction methods can confidently be expected in the future, but it seems unrealistic to expect that one day it may become possible to compute an accurate weather almanac for the following year, similar to the Nautical Almanac. The inherent instability of atmospheric motions seems to rule out this possibility.
Artificial satellites represent the latest application of modern technology to the study of the weather. The first of these was the American Tiros I, launched from Florida in 1960. Orbiting the earth about 14 times a day, it transmitted several hundred televised pictures daily, showing in surprising detail the distribution of clouds as seen from a height of about 400 miles. Storm centres could be easily identified from their cloud pattern, and it was possible to detect incipient tropical cyclones over the open ocean some days before their existence could be inferred from the scanty reports received from areas such as the tropical Atlantic and Pacific. Tiros I and its successors have provided a vast amount of research material which can be expected to yield results of considerable importance for weather forecasting. They are very costly devices, however, and it remains to be seen whether the results will justify their continuance on a routine basis, unless possibly by an international sharing of the costs.
by Neil George Robertson, M.SC., Assistant Director (Climatology), Meteorological Service, Department of Civil Aviation, Wellington.